Add support for current versions of yubikey and devise gems

Couldn't get any tests running in this gem, so I have implemented this change via this test project for validation purposes:

https://github.com/wndxlori/rails3-bootstrap-devise-cancan/tree/yubikey

Given that Yubikey auth now requires an api id and secret key for validation, I also got the yubikey gem guys to update the gem to accept a configuration initializer like:

https://github.com/wndxlori/rails3-bootstrap-devise-cancan/blob/yubikey/config/initializers/yubikey.rb

So you can set the default API_ID and API_KEY via environment variables, instead of trying to pass them all the way through.

I also renamed your 2 added User database columns to follow the Rails snake_case convention, just because it was driving me nuts. I'm OCD that way.